1. Field of the Invention
The present invention relates to a method of dividing a wafer by forming alteration regions in the inside of the wafer by laser beam machining.
2. Description of the Related Art
As a method for dividing a wafer such as a semiconductor wafer or an optical device wafer along streets, there has been tried a method in which a pulsed laser beam capable of being transmitted through the wafer is utilized. In the dividing method based on the use of a pulsed laser beam, the wafer is irradiated with the pulsed laser beam having such a wavelength as to be transmitted through the wafer, from one side of the wafer and while adjusting the beam condensing point to a position in the inside of the wafer, whereby alteration regions are continuously formed in the inside of the wafer along the streets. Thereafter, an external force is applied to the wafer along the planned dividing lines where material strength has been lowered by the formation of the alteration regions, to thereby divide the workpiece (see, for example, Japanese Patent No. 3408805 and Japanese Patent Laid-Open No. 2005-28423).
In the machining by use of the pulsed laser beam, the positional relationship between an irradiation head for irradiation with the pulsed laser beam and an imaging means for imaging the surface of the wafer may be varied relatively during machining, depending on temperature or the like which varies in the process of machining. Therefore, during machining, whether or not the position of actual machining coincides with the position of machining intended has to be checked through imaging by the imaging means.